Or Else
by AgathaBrine
Summary: Bill and Mabel make a deal leaving them trapped in the same body, and Bill intends to blackmail Dipper until he gets what he wants. Probably BillXMabel. Rating subject to change. Mild gore warning. Potential but not likely a sexual content warning for future chapters. You've been warned! Written for Barbacar! :)
1. Chapter 1

After Weirdmageddon, Gravity Falls had returned to its typical state of abnormality. Strange creatures and unexplainable phenomenon continued to happen, but there was an absence of demons that was welcomed by the townspeople. Dipper and Mabel still visited each summer, and by the time graduation came the two had arranged for permanent residence with their Grunkles in the Mystery Shack. Now, at 18 years old, Dipper found himself researching the odd occurrences of Gravity Falls with their Grunkle Ford, while Mabel continued to fight the age-old battle of deciding what to do with her future.

Pulling a plastic bag from his backpack, Dipper dropped to his knees beside the stone statue of Bill to collect a mushroom sample. This year the vegetation hadn't grown around the stone as it had in the past, even with the constant drizzle over the past few weeks. The absence of growth left an uneven - and frankly unattractive - ring of dead grass and dirt around the stone. Although it was more likely that this was being caused by a fungus or bug problem, Ford had asked Dipper to collect some samples of the remaining fungi in case this anomaly had anything to do with Cipher's mischief.

Frankly, Dipper was certain this was the same fungus that was growing behind the Mystery Shack that Grunkle Stan had covered with a large statue of the Multi-bear so that he didn't have to invest in a fungicide.

Plucking a mushroom from the soil gently, Dipper dropped it into the baggy with the others that appeared to be of the same species.

"If only Bill knew that his grave was becoming a fungal paradise" Dipper chuckled, "I bet he'd throw a fit."

"Well,"

Dipper heard a voice behind him, and he shot to his feet in surprise - that was definitely Bill's voice.

"now that you mention it, you guys have been slacking off with the weed control around here, too." A small yellow triangle was comfortably perched in the air behind Dipper, floating as if seated in an invisible chair, "But don't worry, I took care of it!"

Dipper glanced around them nervously – why weren't they in the dreamscape?

"I thought you were dead." He eyed Bill cautiously as he pulled himself to his feet.

"And I thought you were twelve!" Bill laughed, his eye curling up at the edges like smiling lips, "But boy, do things change fast."

Dipper heard leaves rustle behind him, and he craned his neck to see Mabel step into the clearing behind him,

"Sup Bro-bro!" Her smile suggested she had more to say, but the words died on her tongue when she spotted the demon. Her eyes widened and she gasped,

"But I thought…"

Bill cut her off with a hearty laugh, "What, that I was dead? It's almost like you two are the same person! Funny that you're here too Shooting Star, 'cause I have a proposition for you."

"Whatever it is, I'm not interested." She scowled and straightened her posture, "there's nothing you can offer me that I want badly enough to set you free."

"Oh I don't want to be free," Bill snapped his fingers and the tree behind her exploded into flames, sparks flying past her as she yelped and sprung away from the heat, "not yet."

The rain fizzled from the heat, and steam rose into the air, mixing with the dark smoke.

"Run Mable!" Dipper grabbed his backpack and darted towards her, reaching for her arm. A loud crack split the air and the burning tree plummeted to the ground between them, barely scratching along Dipper's extended fingers as it fell. He pulled his hand back with a shocked cry and leapt away from the tree. The damp grass smoked momentarily before catching fire. With the wave of his hand, Bill engulfed another tree in fire. Dipper could hear Mabel calling out for him, but the roar of the flames drowned out her words.

"Get to the Mystery Shack!" Dipper bellowed, but he didn't have time to see if she had heard him.

Smoke was billowing around him, and the fire surrounded him. He knew he had to escape, but the smoke overwhelmed him, choking the air from his lungs.

Mabel was on the other side of the fire, free to run to the Mystery Shack, but she refused to leave Dipper behind. She couldn't see past the smoke, but she knew he couldn't be far. With a burst of courage, Mabel pulled her sweater over her mouth and plunged into the flames. The heat licked her cheeks, and she pulled her hood up for protection. She could hear Bill laughing beside her, but she didn't spare him a glance. Her foot hit the trunk of the tree that had landed between them, and she tripped, dropping her sweater from her mouth as she split her palm open in an attempt to catch herself. She didn't have time to recover - she could feel the flames burning along her legs. She lunged forward once more, head spinning from the lack of oxygen. Her hand hit something soft, and she knew it must be Dipper's arm.

"You know Shooting Star, you there might not have a lot of time left for your goodbyes." Bill's voice remained beside her, but she didn't care.

Her eyes were stinging from the smoke, forcing her to keep her eyes shut as she pulled on Dippers arm, but he wasn't responding.

"Bill, stop it!" Mabel pleaded, helplessly trying to pull Dipper's arm around her shoulders.

 _Why isn't he moving?_

Bill laughed, twirling his cane to clear the smoke around him, "I could, but I thought you didn't want to make a deal."

The smoke filled her lungs with each breath, and she couldn't force herself to speak. With each cough she inhaled more smoke, and she felt her strength draining quickly. She dropped Dipper back to the ground and pressed her palms to her temples in an attempt to clear her mind.

"You know, I could make this all go away."

She didn't look up. She didn't know where Bill was, but a breath of clean air suddenly filled her lungs. Tears pricked at the edges of her eyes as she tried to lift Dipper once more in vain. Her hands began to shake and her tears dried on her cheeks before they fell.

Everything was happening too fast.

"I just need a new meat puppet for a little while. What do you say?" Bill's voice was relaxed and teasing, "I'd even let you stick around while I'm here! Looks to me like there might be room enough for the two of us." He leaned forward, pressing a small hand to the side of her face.

For a moment, the flames seemed to stop burning, and Mabel forced her eyes open.

She would do anything for Dipper. _Anything._ She had to do this.

"Time's running out!" Bill leaned back to extend his free hand to her, a blue flame engulfing it.

Mabel didn't have time to weigh her options. She had to save Dipper. Her watery eyes locked with Bill's and she grabbed his hand. She felt the flames engulf her completely, and for a moment she was certain she was dying. A second later the fire began to dissipate, slowly dispersing until it died out completely. Bill was gone, and Dipper lay beside her, still unconscious, but alive.

Mabel glanced around them, exhaustion rushing over her. She dropped her head onto Dipper's chest as she tried to convince herself to run for help. The world was spinning, and she was still coughing on each breath. Her lungs burned and the ground around them was a charred black. The smell of burning wood and fabric filled her nose. Her whole body felt as if it was made of lead, and she didn't have the strength to pull herself to her feet. Instead, she watched the dizzying world around her spin and fade until everything turned black.


	2. Chapter 2

Mabel awoke to the soft, rhythmic beeping of the hospital monitors around her. Dipper lay in the bed beside her, still unconscious. There was a dull pain in her temples and her entire body was stiff. She stayed in bed for while, watching Dipper sleep while waiting for her groggy thoughts to become coherent. After some time, a nurse stepped into the room check on the two, and a short conversation later Mabel had the necessary tubes and equipment attached to a portable stand. She excused herself to the washroom as soon as possible to stretch her legs and check out her injuries in the mirror.

In the washroom, Mabel immediately gravitated towards the mirror; her skin had grown pale during her hospital stay, but there was no visible burns or bruises. She leaned in close to examine the large bags beneath her eyes. Her greasy hair clung to her neck uncomfortably and washing her face in the sink made little difference. She rested her forehead on her forearm and took a deep breath, her mind slowly retracing her most recent memories. She could remember the fire and…

 _Heya there, Shooting Star. I see you've come to! I was beginning to think you'd sleep forever._

Bill's voice echoed through her mind, unmistakeable and overwhelming.

"Oh no… oh no no no…" She squeezed her eyes shut and shook her head.

How could she have forgotten?

 _Don't be so down! We're going to have a grand time together. Look what you can do now, sharing this sack of meat with me._

The soap lifted from beside the sink and was flung haphazardly across the room. The bar made a gross _splat_ as it hit the floor.

 _But first thing's first: I'm gonna have to have a little chat with your brother._

She pictured Dipper, helplessly asleep in the room beyond the bathroom door. How could she protect him now? How could she have been so irresponsible to make a deal with Bill? There must have been a better way, if only she had had more time to think before…

Her arm moved on its own to carefully swipe a strand of hair over her ear, and she gasped, latching onto the wrist with her other hand. How much control did Bill have?

"You don't touch Dipper!" she summoned all her courage, but a part of her already knew there was little she could do to interfere.

 _You don't call the shots around here anymore, sweet cheeks._

A spasm caused the muscles in her possessed hand to dig her nails into her cheek. She gasped and struggled to pull it from her face, but Bill's control was stronger than her own. She struggled in vain for another few moments before it was lifted from her face and dropped to her side lifelessly. She had regained control of it again… for now. She leaned into the mirror, gripping the faucet with both hands. There were deep red crescents where her nails had dug into her skin, but she hadn't drawn any blood.

Bill cackled from inside of her, lifting a foot from the floor before dropping it in a seemingly playful manner,

 _You and I are going to have so much fun!_

A depraved grin slowly spread across her face in the mirror, and for a minute she wondered if any of this was real. She didn't recognize her reflection as herself with such a malicious expression. Bill was messing with her, she knew that. He was freaking her out on purpose to demonstrate his power, to scare her into submission. But even though she knew that, it was still working. She ran her hands through her hair, eyes locked with those in the mirror. She backed away slowly, trying to tear herself away.

 _How about we go have that chat with your brother now?_

She felt her back hit the wall of the small bathroom, and tears welled up in her eyes as the smile faded. Within a second her expression was that of her own again, mouth turning down at the edges and lip puffing out slightly as tears began to pour down her cheeks. Her lips quivered and she sunk to the floor in defeat. She had gotten not only herself into a world of trouble, but Dipper too.

When she exited the bathroom, she kept her eyes trained on the floor to hide any peculiar expressions Bill may have contorted her face into. She rubbed her hand with her cheek, skeptically massaging the muscles – they were relaxed for now. She pulled herself back onto her bed, watching Dipper's chest rise and fall gently.

 _Aw! He looks like a baby._

He did. He always looked so relaxed when he slept. It was a pleasant change from watching him study, where his brow would furrow and he'd pull his lips into a thoughtful but tight line. He was always too stressed.

Her body lifted from the bed gracefully, but unintentionally. Bill walked her over to Dipper's side, and he leaned into Dipper's shoulder, nuzzling in an affectionate, but unnervingly animalistic manner. Bill moved her mouth to Dipper's ear and took a deep breath, inhaling Dipper's scent. His musk was stronger now that he hadn't bathed in a few days and the faint smell of fire still clung to his hair.

"Mable, you're up!" A deep voice called from the doorway.

Startled her body collapsed to the ground. Bill had given her back control without warning. Within seconds, both of her Grunkles were beside her,

"Are you alright?" Grunkle Stan grasped her hand gently, his brow furrowing with concern.

"We shouldn't have surprised you like that." Grunkle Ford was on her other side, a hand on her shoulder, "Can you stand?"

"I'm sorry, I… I'm alright."

The two helped her back into her bed, supportively,

"Should you be up and about already?" Stan asked, looking her up and down skeptically, "maybe you need more rest."

The two fussed over her for a few more minutes, arranging her pillow and bedsheet around her comfortably and tilting the head of her bed until they were convinced that she was content. A weary voice distracted the three from across the room,

"Grunkle Stan? Ford?" Dipper sounded exhausted, but Mabel was relieved to see him coming to.

How much did Dipper remember? He must have been unconscious before Mabel had made the deal with Bill. Bill had kept his word though – Dipper was alive.

 _I wouldn't tell anyone about me if I were you, Shooting Star. It would be a shame if I had to hurt you so soon after you've only just woken up._

Mabel watched thoughtfully as the grunkles told Dipper about the 'forest fire'. There must still be some way for her to tell everyone what was going on. Grunkle Ford would know what to do. Each thought however was coupled with Bill's snide and threatening comments. He could hear her every thought and every emotion. It made her feel _dirty_.

The rest of the visit was short, but it was long enough to drain all of her energy. The grunkles fussed over the two for what felt like an eternity before the nurses brought the twins a food tray each. Bill was constantly in the back of her mind, and each time she forgot about him for even a second Bill would take control of an ambiguous limb to play with. A few times he would scratch her nose, and once he popped an inquisitive finger into her mouth. He wanted her to always remember that he was there, and _he was in control_. She strongly considered telling her family about Bill. If she could just spit it out, there would be nothing Bill could do to take it back. However, she only got as far as to open her mouth before Bill took control, leaving her gaping like a fish for a moment before regaining composure.

As far as everyone but her and Dipper were concerned, this whole fiasco was caused by a forest fire. The twins had been in the wrong place at the wrong time, and everyone was just happy they had survived. Dipper himself seemed to only have a vague memory of the entire thing, and part of Mabel hoped that he would recall enough to say something to Stan or Ford before Bill could threaten him into silence. There was no such luck.

After an hour of visiting, Grunkle Stan and Ford excused themselves to let the twins get some rest. They gave each twin a goodbye hug on their way out, during which Bill took the opportunity to snake her hand into Ford's coat pocket and retrieve his butterfly knife. Mabel had fought with Bill for a moment, attempting to loosen her grip on the knife, but it was futile. It was as if Bill had humored her by even giving her a chance at control. When Ford tried to straighten up, Bill pulled him back into the hug by his coat collar, giving him another second to get the knife. Something in Mabel's expression must have conveyed her frustration when she let go finally let go, but luckily Ford interpreted this as the result of spending another night in the hospital, and reassured her they would be back the next day to see when the two would be ready for discharge.

Once the two were gone however, Mabel was no longer protected by the need for secrecy, and Bill was free to possess her as he pleased. The door had barely latched behind the grunkles before her body spun on its heels, dropping the knife from behind her back to the bed as it advanced on Dipper,

"Well well well well, Pine Tree. Long time no see." Her face curved into an unsettling grin.

Dipper jumped, eyeing Mabel suspiciously from his bed. His eyes filled with dread and recognition and he looked Mabel over quickly, forcing himself out of bed to back away from her.

"Bill?" A look of uncertainty crossed his face.

"The one and only!"

He backed Dipper up against the wall and planted a hand on either side of him face,

"Boy, have I ever missed you, Sapling." Mabel's head titled slightly, "I have a job for you. Go talk to Fordsie and see what he knows about unbinding spells. I'm sure he's done some research on it before."

Dipper's brow furrowed, "Leave my sister alone and I'll consider it."

Bill tossed the body's head back and cackled loudly, "you think you can bargain with me, Pine Tree?"

He twirled away from the wall and skipped back to the bed to retrieve the pocket knife.

Dipper's eyes widened, "Where did you get that?" he sputtered.

"Ol' Sixer isn't as careful as he should be." He twirled the knife carelessly between his fingers, "but that doesn't matter right now. You want to bargain, let's bargain."

The edges of Mabel's lips strained as the grin bared her teeth, "How about…" Bill flipped the knife open and pressed the blade to her thigh, just below the hospital gown, "we forget Fordsie, and you can watch me skin Shooting Star? I mean the nurses would-"

Dipper lunged at Bill, and grabbed Mabel's wrists, "Let go!"

Bill was laughing, and he fell back onto the bed, pulling Dipper with him. The bed creaked beneath their weight, and Bill easily kicked Dipper between his legs with all the force Mabel's exhausted body could manage. Dipper gasped and instinctively curled in on himself, releasing Mabel's wrists. Immediately, Dipper saw his mistake and terrified eyes met Bill's laughing and crazed ones. Bill had already pushed the knife into Mabel's side, blood spilling onto the bed and seeping into the hospital gowns.

Dipper reached for the knife again, and as his hands wrapped around Mabel's he heard her release a shaky breath. Her hands loosened around the knife, and Dipper looked up to see his sister's teary eyes staring back at him.

"Dipper…"

"Oh my god… Mabel hold on, I'll get a nurse."

Bill had relinquished control again. Even just for a moment, Mabel was back.

"No Dipper, don't let him do this. We can fight him." She winced and inhaled sharply.

Dipper climbed down from her bed, tears pricking at his own eyes as well, "I'll be right back, don't-"

Mabel began to cackle again, the remaining tears falling from her eyes. Bill ripped the knife from her side and sat up as if it was nothing,

"Have you rethought that deal, Sapling?" he lifted the knife thoughtfully to Mabel's fingers and tucked the knife beneath her middle nail to scrape out the dirt.

"STOP! Don't hurt her. I'll do it!" Dipper's voice cracked, "I'll talk to Ford."

"And have you considered what would happen to your precious sister if you told anyone about this little arrangement? You know I'd find out about it."

"I won't tell." Dipper's gaze dropped to his feet, "just let her go, Bill. She needs help."

Bill laughed once again, "Does she ever! Here, something to remember our agreement by."

And with that he torqued the knife beneath her nail, lifting it from the skin. A quiet crack bounced around the room as the nail split down the middle, and Bill grabbed the end with her teeth. He roughly yanked Mabel's head and hand apart, ripping the nail from the flesh. He spat the nail towards Dipper,

"Don't fuck this up kid."

Blood ran down Mabel's finger and a few drops had splattered across her face. The bloody nail sat on the floor between them, causing Dipper's stomach to churn uncomfortably. He darted out of the room as fast as he could, not looking back when he heard Bill laughing behind him.


	3. Chapter 3

After the fiasco in the hospital, Mabel was put on suicide watch and started on a low dose antidepressant. Dipper, however, had been released the next day. Both Grunkle Stan and Grunkle Ford had been angry, confused, and worried, but when Ford realized that Mabel had taken his knife, he turned livid. Stan had to pull him aside to calm him down before Mabel was dealt the entirety of his wrath. It was all out of concern of course, but it certainly would not have come across that way. Grunkle Stan had tried to comfort her by telling her that Ford was just afraid on her behalf, but it did little to ease her mind. When asked why she had done it, she had no idea what to tell everyone, and Dipper was just as clueless. The didn't try to change anyone's minds about her 'suicide attempt' out of lack of explanation, but that didn't make it any easier for Mabel to swallow her new antidepressants.

Dipper was also worried about Mabel, but not for the same reasons as everyone else. Once he was out of the hospital he began to cling to Ford like glue. Dipper had intended to appear as natural as possible, but his nerves made it impossible. He was in too big of a hurry to help Mabel.

"We haven't spent this much time together in years, Dipper." Ford commented casually, assembling something on the counter of his laboratory with Dipper on the chair beside him, "and don't misunderstand me, I love having you around, but shouldn't you be getting back to work on your fungal project outside?"

Dipper shuffled his feet awkwardly, "I… er… The forest… you know."

Ford would think that Dipper was afraid of returning to the forest, and that would be fine. He knew nobody would want to press him for answers if they thought he was still recovering from the forest fire, but he felt bad about lying. It was easier to let Ford come to his own conclusions.

"Ah." Ford looked uncomfortable for a moment as if he wasn't sure if he should offer comforting words or not, "Well all in good time."

Ford turned back to his work, and Dipper began to try and make himself useful by organizing the counter in front of him. A minute of silence passed before Ford cleared his throat,

"Dipper, did something happen… during that fire?"

Dipper tensed, did Ford know?

"Why do you ask?"

"Well… For Mabel to be this upset. I know depression is different for everyone, but she never seemed like the suicidal type. I was just wondering if you knew something. You two are very close."

Dipper contemplated, his eyes trained on the cold coffee mug on the desk. Would Bill really know if he told Ford anything? He glanced at Ford sideways,

"Grunkle Ford…" he slowly looked around the room, but he wasn't sure he was looking for.

Did Bill have other demons watching him? When he looked back at Ford, he realized he had been watching him oddly.

"Do you know anything about unbinding spells?" he awkwardly changed the subject.

Ford certainly noticed, but Dipper had to play it safe. It was too soon to risk telling anyone. He would tell Ford when he was sure they were safe, but for now he would get some information to placate Bill with. Maybe he could figure out Bill's plan in the process.

Ford followed where Dipper's eyes had circled around the room before casting him a suspicious look,

"I did a little research on them back in my university days. Why do you ask?"

"I'm just curious."

"Well," Ford turned back to his work, "any spell used to attach a demon to an object is a binding spell. Do you remember Weirdmaggeddon? We used a binding spell when we banished Bill from Stanley's mind – when we had wiped his memories. The memory-erasing mechanism actually binds memories inside the tube that they are placed in, except with Bill his energy was too strong, and we needed something larger. That's why we have the stone out front. But I digress; an unbinding spell is anything that would break the original binding spell. Each unbinding spell is uniquely related to the binding spell it's reversing."

Oh, so that's what Bill was after. Was he bound to the rock before? How was he now possessing Mabel then?

"Are binding spells permanent?"

Ford stopped fiddling and turned to face Dipper again, "No, very few things are. Of course, it's much easier to transfer the binding to another object than to break the binding spell. But Dipper, Cipher is gone. You don't have to worry about him anymore – he can't hurt us."

Dipper looked at the floor; he had to tell Ford about Mabel.

"How are you so sure?"

There was another long silence, and Dipper finally looked up. Ford was watching him, his expression unreadable.

"Dipper, what started the fire?"

Dipper's heart skipped a beat, "I… I don't know."

Why was he such a terrible liar?

Ford sighed, and moved to beside Dipper, setting his hand on his shoulder, "You don't need to be afraid of Bill. And if anything were to happen – not that it will – we'll deal with it then. Both you and Mabel have grown a lot since then."

Dipper nodded. He shouldn't push this anymore right now. Ford was already getting suspicious. Maybe that wasn't a bad thing, though.


End file.
